With fuel cell systems, in order for a fuel cell stack to initiate electric power generating reaction, a probability arises in which, due to water being required as medium for causing ion transfer through an ion-exchange film, fuel gas and oxidizing gas are supplied under a moistened condition.
With such a fuel cell system, ideally, it is desired for fuel gas and oxidizing gas to prevail under moisture of 100% with no occurrence of excessive water. However, in an actual practice, owing to an excessive amount of moistening water or resulting water accompanied by electric power generating operation of the fuel cell stack, it is highly probable for moisture to excessively prevail in a gas flow passage inside the fuel cell stack. As the amount of moisture occurring in the gas flow passage increases, a difficulty is encountered for gas to permeate through the ion-exchange film, causing a decrease in a cell voltage with deterioration being caused in an electric power generating efficiency of the fuel cell stack.
Japanese Patent Application Laid-Open Publication No. 2001-307757 discloses a structure wherein, when a minimal cell voltage is excessively lowered with respect to an average unit cell power output amount of a fuel cell stack, the discrimination is made that an excessively increased amount of moisture prevails in a gas flow passage of the fuel cell stack, thereby executing purging of fuel gas through the use of a purge valve connected in communication with the fuel cell stack.
Further, with the fuel cell system, although the fuel cell stack generates the electric power output accompanied by generation of heat, since an operating temperature for enabling a solid polymer film to efficiently generate electric power should prevail in a limited range, the fuel cell stack should be kept within a given temperature range by permitting coolant to flow through the fuel cell stack. Here, when considering the occurrence of the greatest increase in the gas temperature within the fuel cell stack, in order to lower the gas temperature, a coolant flow passage is required to be located in an area possibly closest to the gas flow passage to allow coolant to flow near the gas stream. To this end, a structure may be provided that allows fuel gas and coolant to flow in adjacent flow paths separated from one another via a porous film.
Furthermore, with the fuel cell system, in a case where the fuel cell stack is installed on an automobile, there are many probabilities in that coolant is cooled with a wind stream passing through a radiator located in front of the vehicle and supplied to the fuel cell stack.